1. Field of the Invention
The invention relates generally to the field of electromagnetic induction formation evaluation devices. More particularly, the invention relates to methods and systems for determining a direction to a target Earth formation from a downhole instrument disposed in a wellbore by using multiple directional component electromagnetic induction measurements.
2. Background Art
Electromagnetic induction formation evaluation devices known in the art include axisymmetric instruments. Axisymmetric instruments have one or more solenoid-type transmitter coils coupled to a source of alternating current or switched current, and one or more solenoid-type receiver coils disposed at axially spaced apart locations from the transmitter along the instrument. The receivers are coupled to various circuits in the instrument which detect amplitude and phase of voltages induced in the receivers. The amplitude and phase of the voltages are related in various ways to the electrical conductivity of the media (Earth formations) surrounding the instrument. One such instrument is described, for example, in U.S. Pat. No. 5,841,281 issued to Beard et al.
In axisymmetric instruments, the solenoid-type coils are wound so that their longitudinal axes (and consequently their magnetic dipole moments) are substantially parallel to the axis of the instrument, or more particularly, are substantially coaxial with the axis of the instrument housing. As a result, axisymmetric induction instruments have a response which is substantially uniform in any azimuthal direction around the longitudinal axis of the instrument. The transmitter and receiver solenoid coils may be positioned at particular axial spacings from each other, and may be electrically interconnected in such ways (e.g., in series in the same polarity and/or in series in opposed polarity) as to provide the instrument with selected lateral (radial) resolution and with selected axial resolution. Methods are well known in the art for determining a spatial distribution of various electrically conductive media with respect to the instrument, where the instrument includes at least a plurality of receivers. See, for example, U.S. Pat. No. 5,703,773 issued to Tabarovsky et al.
More recently, various types of multi-axial or multi-directional induction instruments have become known in the art. Multi-directional induction instruments include transmitters and receivers which have magnetic dipole moment oriented along directions other than parallel to the axis of the instrument. One example of such a multi-directional instrument is described in U.S. Pat. No. 5,781,436 issued to Forgang et al. The instrument disclosed in the Forgang et al. '436 patent includes transmitters and receivers oriented along three mutually orthogonal axes. The instrument disclosed in the '436 patent has as one objective determining horizontal and vertical components of conductivity of electrically anisotropic Earth formations. Such electrical conductivity components are more generally known as components of electrical conductivity in a direction parallel to the attitude (bedding plane orientation) of the Earth formations, and in a direction perpendicular to the bedding plane orientation of the Earth formations. Determining the parallel and perpendicular conductivity components is performed in general by interpreting measurements of induced voltages made along mutually orthogonal axes, resulting from electromagnetic fields induced along mutually orthogonal axes. One method for determining conductivity in parallel and perpendicular directions in anisotropic Earth formations is described, for example, in U.S. Pat. No. 5,999,883 issued to Gupta et al.
Another example of a multi-directional electromagnetic induction instrument is disclosed in U.S. Pat. No. 5,757,191 issued to Gianzero. The instrument disclosed in the '191 patent is adapted to electronically “rotate” induction signals detected by the receivers on the instrument to recreate the response that would obtain if the instrument were disposed in earth formations having bedding orientation substantially perpendicular to the axis of the instrument. As is well known in the art, having bedding orientation substantially perpendicular to the axis of the instrument is a substantially ideal condition for induction logging using conventional axisymmetric induction well logging instruments because the formations disposed around the instrument would in such orientation tend to have conductivity that is symmetrically distributed around the longitudinal axis of the instrument.
It is often the case, however, that Earth formations are not symmetrically distributed around a wellbore. As described in the '191 patent in particular, and also in the '883 patent referred to above, it is frequently the case that the bedding orientation of the earth formations is not perpendicular to the instrument axis. Bedding orientation other than perpendicular to the instrument axis results from various combinations of formation “dip” (inclination from horizontal of the bedding orientation) and wellbore inclination (deviation of the wellbore from vertical).
It is often the case that the Earth formations are not arranged in substantially planar, parallel arrangements of various layers of the formations as is commonly simulated or modeled in order to interpret response of common well logging instruments. In some cases, an Earth formation of interest may consist of an isolated (laterally and/or vertically limited) segment of rock which has a different electrical conductivity than the surrounding Earth formations. Some of these isolated formations of interest may be petroleum bearing reservoirs or other formations of economic interest as they can be used as marker formations in directional drilling. In order to more efficiently be able to penetrate such isolated, or “target”, formations of interest, it is desirable to have a wellbore instrument that can estimate a direction to the target formation from any position within a wellbore.